In order to overcome the limitations in high speed operation in a large scale integrated circuit (LSI) with electrical wires, faster signal communication between chips has been attempted by forming an optical circuit on an Si substrate. The optical circuit is formed of optically functional elements such as an optical modulator for modulating an input signal, an optical receiver for receiving signal light and a splitter for branching light. Light is input to these optical functional elements via an optical waveguide for propagating an optical signal.
When such an optical circuit is formed on a semiconductor substrate, light is reflected from connections between an optical waveguide and an optically functional element due to the effects of a mode mismatch caused by a mismatch in the refractive index. The reflected light is led along the optical waveguide in the reverse direction and, thus, noise is generated. Light is reflected from portions other that the portions connected to an optically functional element, for example, from a terminal of an optical waveguide, which causes noise.
In order to reduce light reflected from optical connections that may cause noise in an optical receiver as described above, it has been proposed to provide a diffraction grating reflecting mirror that operates as a filter directly in front of the input terminal of the optical receiver (see Patent Document 1). It has also been proposed to cut an optical waveguide diagonally relative to the direction in which an optical signal propagates in order to structurally prevent a terminal from allowing an optical signal to return in the direction in which the optical signal has been propagated (see Patent Document 2).